1. Field of the Invention
The invention relates generally to water conductivity monitoring circuits and, more particularly, to a circuit that uses a microprocessor for monitoring water conductivity in a steam generator.
2. Description of the Related Art
Steam generation is found in home appliances for a variety of different uses. For example, it is known to use a steam generator in an oven for cooking applications. In a steam generator, a water source typically supplies water to a boiler to generate steam. For a steam generator in an oven, water can be supplied from a reservoir and pumped into the boiler, or directly from a continuously pressurized water source such as a municipal water supply.
Most common sources of water leave calcium and magnesium deposits in the boiler after the water is vaporized into steam, a cumulative build up of which adversely affects performance. One solution to the build up of deposits in a boiler is to add a cleaning solution to the water source that will dissolve the deposits, and then flush the effluent through a drain. A more common and practical solution in home appliances is to limit the dissolved solids that can reach the boiler by using an ion exchange filter upstream of the boiler. An ion exchange filter typically removes 99% of all dissolved solids from the source water, leaving essentially pure water for steam generation in the boiler.
The principal problem with a filter, however, is that it must be replaced periodically when it becomes saturated with solids or when it otherwise breaks down and its usefulness expires. There is a need to determine when to replace a filter in a steam generator.
It is known to measure the purity of water by measuring its conductivity value, since the conductivity of water is directly proportional to the quantity of ionizable dissolved solids found in the water. U.S. Pat. No. 4,496,906 to Clack discloses a device for continuously monitoring the electrical conductivity of a liquid. The device includes a housing with parallel-spaced electrodes for insertion into a liquid, and a transparent user-viewable lens. The electrodes are connected within the housing to a differential amplifier which provides a change in output signal level when the liquid conductivity exceeds a predetermined threshold level. A pair of LED's of different colors connected between respective unidirectional current sources from the output of the differential amplifier and viewable through the lens indicate acceptable and unacceptable conductivity levels of the water.
But, assessing the purity of water by measuring conductivity carries its own set of problems. For example, introducing an electrical current from a probe changes the very chemistry of the water to be measured. As well, probes get contaminated with deposits that affect their sensitivity. Further, known water purity conductivity devices provide only a “thumbs up” or thumbs down” assessment, measured against a fixed threshold. Either the water meets a standard of purity or it does not.